I have a 1930's house (no felt under the original concrete tiles). I want to put stuff in the loft, but it gets covered in ***p. So, I've screwed 5mm ply to the inside face of the 4 x2 rafters, I have now run out, so have used triple wall cardboard, I'm sure all this lot must be moisture permeable, but I'm only a humble DIYer. I will leave a gap of 6" alround at the bottom for ventilation.............is this OK or am I heading for problems?